1. %SD can be used to determine the amount of error in a single value
1. This can be applied to the number of counts acquired from a well counter or uptake probe
2. As you increase the number of counts acquired you reduce the percent error or the %SD decreases
3. This tells us that the more counts you have the less error occurs
2. Formulas are noted below for 1 SD, 2 SD, and 3 SD (N = counts)



3. Application



4. From the calculations above note that when you increase the counts you reduce the percent error
1. The formula above uses only 1 SD
2. As a general rule, it can be stated that you always want to acquire at least 10,000 counts in any sample. The reason for this is that at 1SD there is only a 1% chance of error. If you apply 3SD then the there would be a 3% chance of error.
3. In addition, you should note how your %SD decreases as you increase in the number of counts in a sample
5. Homework
1. What is the percent error in a thyroid uptake that has 254,000 counts at 3SDs?
2. What is the percent of error in the thyroid uptake that has 25,000 counts at 3SDs?
3. Of the two mentioned, which has the least amount of error?

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